This invention generally relates to monitoring a condition occurrence on a first member moving relative to a second member. More specifically, the invention provides a method and apparatus for improving and/or extending the operating range of an electronic monitoring device such that the changes attributed to temperature, amplification, component values, circuit parameters, initial adjustments, and environmental influences have a minor effect on the ability of the device to operate in its intended application.
There are many and various prior art devices, both simple and sophisticated, that are adapted for the detection of a condition occurrence. These devices are, for the most part, electronically oriented and include a circuit configuration that is adapted to be responsive to the condition occurrence being monitored. In applications involving moving members, it is common practice to utilize the effect of interacting inductances so that physical connections are not required between the sensing circuit on the moving member and the monitoring circuit on the stationary member. Further, it is the practice to establish an operating mode for the monitoring circuit such that the moving member, in its normal condition, has either no interacting effect on the monitoring circuit, or alternately has a marked effect on the monitoring circuit. In either case, there is established in the monitoring circuit, an operating mode or range within which the circuit must operate dictated by the distance between the sensing circuit and the monitoring circuit and/or by the particular circuit, its parameters, and the effect of influences other than the effect attributed to the sensing circuit.
Examples of condition monitoring devices of the type alluded to are found in vehicle tire monitoring systems, conveyor belt rip detection systems and the like. In these type applications, inductive coils are mounted on the wheel or belt in a normally open-circuited or close-circuited state such that an abnormal condition occurrence will effectively change the coil status to its alternate close-circuited or open-circuited state respectively. This change in the sensor coil state due to the condition occurrence on the moving member is used to alter the normal operating status of the monitoring circuit when it passes proximate thereto. This may be accomplished by decoupling a pair of mutually coupled coils associated with an oscillator in the monitoring circuit to interrupt an oscillatory signal, or alternately, to provide coupling of the coils such that a nonoscillatory circuit creates an oscillatory signal. Further, this may be accomplished by increasing or decreasing coupling between inductances to establish an oscillatory or nonoscillatory state respectively. Other variations of this general theme are possible and may be found by reference to prior art patents in the field and to the literature.
While these prior art monitoring systems are basically of sound design, it has been found that, in attempting to apply the devices to the extremely harsh environments of vehicle tire and industrial belt monitoring applications in a miniaturized configuration, various influences tend to alter the operating characteristics of the circuitry and otherwise cause instability and/or erratic operation.
For example, FIG. 1 is a graphical illustration of the operation of a circuit configuration that is adapted to operate in an oscillatory mode. In the drawing, the ordinate of the graph represents change in the functional characteristics (.DELTA.F) of the circuit due to amplification, temperature, component aging, environment and other influences and F.sub.1 and F.sub.2 are the limits established for the circuit in its design for a particular application. An oscillatory circuit is generally defined as a feedback circuit in which the output is coupled back to the input in the proper phase and magnitude to sustain oscillation. In this respect, therefore, the graph illustrates the boundary F.sub.b for the feedback condition wherein the circuit is either an oscillator or a nonoscillator and the shaded area of the graph illustrates the bounded area in which the circuit functional characteristics and the feedback are right for the circuit to operate as an oscillator. Of course it should be understood that the graph is representative of a circuit having a particular combination of operational parameters and that the change in state boundary F.sub.b will vary according to the variance in the circuit parameters. The abscissa or X-axis of the graph represents the other boundary of the effective region of oscillation. To illustrate how the graph is used to describe an oscillatory circuit including an inductive feedback circuit wherein the positive X-axis may be representative of decreasing M, the mutual coupling of the feedback energy, it will be assumed that a particular circuit is configured to operate within the range F.sub.1 and F.sub.2 and that coupling M.sub.o is that minimum coupling which will maintain the oscillatory state of the circuit for a .DELTA.F between F.sub.1 and F.sub.2. As shown by the graph, the circuit operation is within the shaded area and will continue to operate in an oscillatory mode only as long as .DELTA.F does not exceed F.sub.1 or F.sub.2 by an amount to move point P.sub.o outside of the boundaries. Similarly, for a .DELTA.F less than F.sub.2 the mutual coupling M.sub.o may decrease and still maintain the circuit in an oscillatory operational mode. To illustrate by way of an example, should .DELTA.F change to F.sub.3, then for the same M.sub.o the circuit operating point is moved to P.sub.3 and the circuit no longer functions as an oscillator. In order to bring operation back within the bounded area, the mutual coupling must be increased to M.sub.3 and the circuit will resume its oscillatory mode of operation. This discussion presumes that the feedback coupling is a function of the mutual coupling between a pair of inductances (L.sub.1,L.sub.2) in the circuit designated the monitoring circuit; however, if the separation of these inductances is a constant, then the coupling may be a function of a separate influence, i.e., a tertiary circuit including an inductance L.sub.3 in proximate position to L.sub.1 L.sub.2 such that the oscillatory operating mode of the primary or monitoring circuit is affected. If, in this circumstance, a negative feedback coupling is associated with the proximity of L.sub.3, the effective mutual coupling M is decreased. A decrease in mutual coupling of .DELTA.M or more (indicated by the cross-hatched area) will result in a change of state from oscillatory to nonoscillatory along the boundary F.sub.b. If, however, the operational conditions should change such that .DELTA.F is effective to move point P.sub.o to the left toward P', then the circuit no longer can change state with a decrease in coupling provided by L.sub.3 and it would require closer proximity of L.sub.3 to provide a greater coupling change .DELTA.M over the new parameter range.
A similar graphical illustration may be used to demonstrate the operation of a nonoscillatory circuit configuration. Referencing FIG. 2, it should be clear from the above discussion with respect to the oscillatory circuit of FIG. 1 that a nonoscillatory circuit, operating in the shaded area of FIG. 2, may revert to an oscillatory mode by reason of the change in circuit functional characteristics .DELTA.F to put the operation outside of the F.sub.b ' boundary and the X-axis. For example, a change in circuit parameters beyond the range of F.sub.1 or F.sub.2, such as indicated at F.sub.3, will result in the operating point moving to P.sub.3 ' and the initial coupling between L.sub.1 L.sub.2 (at M.sub.o ') will have to be decreased to M.sub.3 ' for satisfactory nonoscillatory operation. When a tertiary circuit including an inductance L.sub.3 is operating to provide increased or positive feedback coupling .DELTA.M' between L.sub.1 and L.sub.2, a change of state from nonoscillating to oscillating is initiated as defined by the F.sub.b ' boundary and within the limits F.sub.1 and F.sub.2.
It should be apparent from this discussion that the operating mode of the monitoring circuit, whether oscillatory or nonoscillatory is a function of a plurality of operational characteristics that establish limits in its operation. It is therefore an object of this invention to provide a method and apparatus for extending the operating range of a condition monitoring system by establishing sensor and monitoring circuit configurations that operate over a broader range of circuit functional characteristics.
It is a further object of the invention to provide a monitoring system configuration that provides an output signal indicative of the sensed condition upon the proximate passage of a closed-circuited sensor circuit regardless of the initial oscillatory or nonoscillatory state of the monitoring circuit. These objects are accomplished in a monitoring system adapted for sensing and indicating the occurrence of a change in condition of a first member moving relative to a second member comprising: circuit means on the second member responsive to a signal at its input to provide signal conditioning and an indication of the normal and/or abnormal condition of the first member; a monitoring circuit on the second member comprising an amplifier having input and output inductances, the configuration of the amplifier being such that it is at a threshold of one or the other of two operational states, the first state being such that the operational parameters of the circuit influence regenerative feedback over degenerative feedback and the amplifier oscillates while the second state is such that the operational parameters of the circuit influence degenerative feedback over regenerative feedback and the amplifier does not oscillate, said monitoring circuit coupled to the circuit means to provide a signal that will influence the status of the circuit means for indicating the change in condition; and an inductive sensor circuit on the first member, movable therewith and responsive to the occurrence of a change in the condition of the first member, said sensor circuit being in a configuration to provide intercoupling of the amplifier input and output inductances when close-circuited and in proximate position thereto such that a signal is generated in the monitoring circuit regardless of its current oscillatory or nonoscillatory operational state.